Bonding film

ABSTRACT

A heat-curable self-supporting organic polymeric bonding film consisting essentially of a single reactive layer that includes a composition comprising (1) 100 parts by weight of heat-curable epoxy resin; (2) a high-molecular-weight flexibilizing polymer that is reactive with epoxy resin and provides 5 to 40 parts by weight of elastomeric segments in the composition for each 100 parts by weight of epoxy-resin segments; and (3) a roomtemperature-latent curing agent. An exemplary composition includes a blend of diglycidyl ethers of bisphenol A; a carboxylated acrylonitrile-butadiene elastomer; and a curing agent combination of substituted urea and dicyandiamide.

United States Patent Pagel 1 July 8,1975

[73] Assignee: Minnesota Mining and Manufacturing Company, Saint Paul,Minn.

[22] Filed: Oct. 31, 1969 [21] Appl. No.: 873,132

Related U.S. Application Data [63] Continuation-in-part of Ser. No.579,495, Sept. 15,

1966, abandoned.

[56] References Cited UNITED STATES PATENTS 2,858,291 10/1958 McAdam260/837 2,879,252 3/1959 Been 260/837 2,947,338 8/1960 Reid 260/8363,100,160 8/1963 Korpman 260/836 3,312,754 4/1967 Marks 260/8373,340,224 9/1967 Sherman... 260/836 3,436,359 4/1969 Hubin 260/8303,496,138 2/1970 Sellers... 260/830 3,580,830 5/1971 Siebert 260/836Primary ExaminerPaul Lieberman Attorney, Agent, or FirmAlexander, Sell,Steldt & DeLaHunt 5 7] ABSTRACT A heat-curable self-supporting organicpolymeric bonding film consisting essentially of a single reactive layerthat includes a composition comprising (1) 100 parts by weight ofheat-curable epoxy resin; (2) a high-molecular-weight flexibilizingpolymer that is reactive with epoxy resin and provides 5 to 40 parts byweight of elastomeric segments in the composition for each 100 parts byweight of epoxy-resin segments; and (3) a room-temperature-latent curingagent. An exemplary composition includes a blend of diglycidyl ethers ofbisphenol A; a carboxylated acrylonitrile-butadiene elastomer; and acuring agent combination of substituted urea and dicyandiamide.

15 Claims, No Drawings BONDING FILM CROSS-REFERENCE TO RELATEDAPPLICATION This application is a continuation-in-part of patentapplication Ser. No. 579,495, filed Sept. 15, 1966 and now abandoned.

BACKGROUND OF THE INVENTION Temperature of test 67F. 75F. l80F. 250F.

Overlap shear (minimum 3500 3500 2000 750 in pounds/square inch) T-peel(minimum in IS 15 15 pounds/inch of width) Honeycomb peel" l l0 (minimumin inch-pounds/inch of width) Beam-creep (maximum in mils) 5O 50 Thefree ends of strips of l-inch-wide. 4-inch-long. 64-mil-thick. 2024 T3clad aluminum alloy sheeting bonded together at their other ends in alinch-ovcrlapping joint are pulled in opposite directions along theirlongitudinal axes. The bonds are formed with a bonding film having aweight of about 0.08-pound/square foot (used in all the tests), and thebonding surfaces of the adherends have normally heen primed by coatingwith a resinous primer (as in all the tests).

The adjacent ends of l-inch-wide, Sinch-Iong. strips of -mil 2024 T3clad aluminum alloy sheeting adhered together over most of their lengthare bent apart at right angles and pulled in opposite directions.

A free end of a 3-inch-wide, IO-inch-long. ZU-mil 2024 T3 clad aluminumalloy sheeting is pulled from the Vzinchthick, Ainch-cell honeycomb coreof 4-mil 3003 aluminum alloy foil to which it is bonded by wrapping thesheeting around a 4-inch-diameter roller riding on the surface of thesheeting.

Three-inch-widc. 8-inch-long laminates of the described honeycomb corewith 64 mil skins of 2024 T3 clad aluminum alloy sheeting are placed onsupports spaced 6 inches apart. A IOUO-pound weight at 75F. and anSOD-pound weight at I80F. are loaded midway between the supports. AfterI92 hours the deformat of the center of the beam is measured.

Sheet-to-sheet bonds and sheet-to-honeycomb bonds have different,competing requirements, and no bonding film has previously beenavailable that reliably provided both kinds of bonds. The requirementsfor sheetto-sheet bonds have generally been met with epoxyorphenolic-based bonding compositions modified by the addition of ratherlarge amounts of flexibilizing materials, such as nylons, polyvinylacetal resins, and acrylonitrile-butadiene elastomers. Theseflexibilized compositions have been required to provide the peelstrengths necessary in sheet-to-sheet bonds.

But the use of large amounts of flexibilizers in sheetto-sheet bondingfilms prevents those bonding films from being satisfactory forsheet-to-honeycomb bonds. The achievement of satisfactory bonds tohoneycomb requires a material that exhibits flow and othercharacteristics during initial heating of the bonding film causing thefilm to wet and fillet along the contacted edge portion of honeycomb.The previous flexibilized bonding films useful for sheet-to-sheet bondshave not exhibited the necessary filleting characteristics. More thanthat, the cured bonds produced by these previous flexibilized bondingfilms have generally been so flexible that they permitted excessivedeformation of the final sheetto-honeycomb structure.

In an attempt to provide a bonding film useful for both sheet-to-sheetand sheet-to-honeycomb bonds, bonding films of two layers, one an epoxycomposition and the other an epoxy-elastomer or phenolicelastomercomposition, have been produced; see Martin et al., US. Pat. No.2,951,004. However, such bonding films have generally not met the neededstrength requirements over the wide range of temperatures set out in thetable above. Further, when forming sheet-to-sheet bonds, the layer ofpure epoxy composition has sometimes caused the bond to fail under peelstresses. Also, care has been needed to assure that the correct side ofthe composite bonding film was palced against the honeycomb, and thisrequirement has caused errors, waste and delay in the productionprocess.

What has been needed is a bonding film that consists essentially of asingle reactive layer that flows sufficiently when initially heated sothat it will wet and fillet on honeycomb and still will exhibit thestrength properties summarized above. But although the two-layer bondingfilms have been sold for many years and their deficiencies recognized, asatisfactory single-layer bonding film useful for both kinds of bondshas never been produced.

SUMMARY OF THE INVENTION The present invention provides the neededsinglelayer heat-curable self-supporting bonding film. The singlereactive layer of a bonding film of this invention includes (a) aheat-curable composition that comprises an epoxy resin which has on theaverage more than one 1,2 epoxy group per molecule; (b) ahigh-molecularweight fiexibilizing polymer that has reactive functionalgroups by which the polymer may be crosslinked to epoxy resins and thatprovides 5 to 40 parts of elastomeric segments in the composition foreach parts of epoxy resin segments; and (c) a room-temperature latentcuring agent that is active at elevated temperatures to initiatereaction of the composition to an integrated state. A preferredflexibilizng polymer comprises a reactive acrylonitrile-butadienepolymer; for example, a reactive acrylonitrile-butadiene elastomer maysimply be mixed with the other ingredients, or a flexibilizing polymermay be formed by prereacting a reactive acrylonitrile-butadiene polymerwith at least part of the epoxy resin in the composition. The filmgenerally is between about 5 and 30 mils in thickness and a scrim-typefabric may be embedded in it. To assure that there is no prereaction ofthe flexibilizing polymer with itself or with the epoxy resin, thereactive layer of the bonding film is essentially free ofroom-temperature-active curing agents.

5. Elastomer" and elastomeric" are used in this specification todescribe substances that can be stretched at room temperature to atleast twice their original length, and after having been stretched andthe stress removed, return with force to approximately their originallength in a short time. vulcanizable elastomer-precursor, or more simplyelastomer-precursor," is used in this specification to mean polymericmaterials that may be crosslinked in a vulcanizing-type reaction toprovide a substance that meets the above description. As an example, forpurposes of this specification, vulcanizable elastomer-precursor andelastomer-precursor" include so-called liquid elastomers, such as liquidacrylonitrile-butadiene polymers. Segment is used in this specificationto mean a molecule or portion of a molecule. Elastomeric segments isused to describe either separate elastomeric molecules or portions ofmolecules that would be elastomers or vulcanizable elastomer-precursorsas described above if they were a separate molecule rather than aportion of a molecule. Epoxy-resin segments is used to describe eitherseparate molecules having one or more l,2 epoxy groups and an averageepoxidc-equivalent weight less than about 5000 or to describe portionsof molecules that have the same structure and composition as theseparate epoxy resin molecules just described.

One reason for the success of bonding films of the invention is thatthey incorporate a flexibilizing polymer that is reactive with epoxyresin. However, bonding films of the invention are not the first toincorporate a reactive elastomeric material in combination with epoxyresin. For example, one previous commercial bonding film included 100parts epoxy resin, 50 parts carboxylated acrylonitrile-butadieneelastomer, and parts of a thermoplastic fiexibilizing polymer. Thisbonding film, however, was designed for and could only be used forsheet-to-sheet bonds, and it did not change the general view thatsingle-layer epoxy-elastomer compositions could not be used for bothsheet-to-sheet and sheet-to-honeycomb bonds. Another prior suggestionoccurred in the Martin et al. patent cited above, where an adhesivecomposition including 100 parts epoxy resin and parts of carboxylatedacrylonitrilebutadiene elastomer was suggested for use as one of thelayers of a two-layer bonding film. But that suggested adhesivecomposition also included curatives for the elastomer, and it is nowknown that, especially under the storage conditions used for bondingfilms at this time, those curatives caused premature reaction betweenthe epoxy resin and the carboxylated elastomer. This premature reactionstiffened the composition, making a film of the composition quiteunsuitable for use in bonding to honeycomb structure. Rather thandispelling the common conception that single-layer flexibilized bondingfilms could not be used for the bonds in both sheet-to-sheet andsheet-to-honeycomb structural members, the prior suggestion in Martin etal reinforced that common conception.

Another type of prior work with adhesive compositions that incorporateepoxy resins and carboxylated acrylonitrile-butadiene elastomers isfound in Marks, US. Pat. No. 3,312,754. That patent suggests a liquidadhesive composition for use to adhere the seams of metal cans, and thecomposition suggested includes the reaction product of a liquid epoxyresin and liquid carboxylated acrylonitrile-butadiene elastomer, as wellas other flexibilizing materials. Because it is liquid, this adhesivecomposition is not useful as a bonding film. But more than that, whenthe composition is tested by the above standard tests for the bonds inhighperformance structural members, it fails to pass nearly all of thestandards. It is now known that a major reason for the inability of thisadhesive composition to meet the strength requirements forhigh-performance structural members is the low molecular-weight of thereaction product of epoxy resin and carboxylated acrylonitrile-butadieneelastomer used in the adhesive composition.

Now, contrary to what the prior experience in the art suggested,applicant has found that bonding films of this invention satisfactorilyprovide both the sheet-tosheet and sheet-to-honeycomb bonds inhighperformance structural members. Bonding films of the invention dobecome flowable when heated to an elevated temperature so that they willwet and form a fillet on the edge portion of a honeycomb pressed againstthe film. Furthermore, after a bonding film of the invention has cured,a bond is formed that does not exhibit excessive creep and meets each ofthe other standards recited in the above table.

Another desirable feature of preferred bonding films of this inventionis that they cure at a temperature of 250F. This low curing temperatureis a significant advantage, first, because it reduces the cost andshortens the time of the curing operation. Further, aluminum, which is acommon structural material, is desirably not subjected to temperaturesabove 250F. because of possible corrosion effects and reduction offatigue resistance.

All in all, bonding films of the invention met an important and largeneed in the structural adhesive industry; since the parent of thiscontinuation-in-part patent application was filed, bonding films of theinvention have largely supplanted the previously long-used twolayerbonding films, and bonding films of the invention have assumed a majorrole in bonding both sheet-tosheet and sheet-to-honeycomb laminatedstructural members.

DETAILED DESCRIPTION The invention will first be illustrated byreference to two exemplary bonding films.

EXAMPLE 1 A bonding film was formed from the following ingredients:

Parts by Weight Epoxy resin (a room-temperaturesolid blend comprisingthe following reaction products of bisphenol A and epichlorohydrin I00Softening point Epoxide Parts by by Durrans Equivalent Weight methodWeight Epon 834 82F. 225-290 63.1 EKRB 2002 l45l70F. 450-525 26.5ERL-2774 liquid at F. 180-195 10.4

with viscosity of 1 1,000 14,000 cps Parts by Weight Carboxylatedacrylonitrile-butadiene elastomer (I-Iycar 1072; aroom-temperature-solid elastomer having a number-average molecularweight of about 30,000 and comprisin about 5% acrylic acid, 35%acrylonitrile and 0% butadiene) 24.5 3-(p-chlorophenyl)- 1l-dimethylurea 3. l Dicyandiamide 6.3 Chrome oxide colorant 0.7 Acetonesolvent 90.0

The carboxylated elastomer was first milled into a sheet in a rubbermill and the sheet then cut up and mixed with acetone in a churn untildissolved. The 3- (p-chlorophenyl)-l l -dimethyl urea, dicyandiamide,and chrome oxide were mixed into the ERL-2774 epoxy resin in two passeson a paint mill. The Epon 834 epoxy resin was heated to about F. andadded, together with the EKRB-2002 epoxy resin, to the carboxylatedelastomer solution in acetone. After this, the materials premixed on thepaint mill were added to the acetone solution.

This mixture was knife-coated in a l3-mil-thick (wet) coating onsilicone-treated glassine paper and dried for 30 minutes at F. Theresulting film was then wound in a roll, with a light, non-woven, Dacronscrim disposed on the exposed surface of the film, whereupon it becameembedded in the film. The resulting film 'had a thickness of about 5mils and a weight of about 0.03

pound/square foo t. Thicker films were also made by laminating;in'onecase, two, and in another case, three, thicknesses of filmtogether, with the liners for the sec- 0nd and third thicknesses removedto give films that weighed 0.06 (about mils thick) and'0.08 (about milsthick) pound/square'foot. v Samples of the 0.08 pound/squarefoot-bonding film were used to adhere aluminumsheets together and toaluminum honeycomb cores for testing in the manner described above. Thealuminum sheets had been etched with a sodium dichromate-sulfuric acidsolution and coated with a resinous primer solution'which'had then beendried to a final thickness of 0.1 mil. The bonding film was placedbetween the sheeting and honeycomb core and the resulting sandwichesplaced'iri'a fixture under about 50 pounds/squareinch pressure. Thebonding film was then cured by heatingat a temperature-rise rate ofabout 4 -8F./minute to'a temperature of 250F. which was held for 60minutes.

After this curing operation, the laminated structures were subjected totests at 67F., 75F., 180F.,'and

250F. following the procedures set out above. The following results wereobtained:

The following example illustrates a bonding film in which epoxy resinand a reactive elastomer-precursor are prereacted to form aflexibilizing polymer.

EXAMPLE 2 -The ingredients were: Parts by y Weight Epoxy resincomprising the following three reaction 100 products of bisphenol A andepichlorohydrin and one triglycidyl derivative of paramino phenol(ERL-0510) Parts by Weight Softening point Epoxide Parts by DurransEquivalent by method Weight Weight Epon 834 82F. 225-290 30 Epon I004l05.C. 875-1025 40 ERL 2774 liquid at 80F. 180-195 15 with viscosity of1 1,000 14.000 cps ERL 0510 liquid at 80F. 97-101 l5 with viscosity of500-700 cps Carboxylated acrylonitrile-butadiene vulcanizableelastomehprecursor Hycar CT BNX (a room-temperature-liquid vulcanizable25 elastomer-precursor having a number-average molecular weight of 3600and including about 20 weight-percent acrylonitrile and 0.07acid-equivalents per 100 grams of material) Substituted-urea reactionproduct of toluene diisocyanate 8 and dimethyl amine Dicyandiamide 4Methyl ethyl ketone solvent 50 65 The Epon 834 and Epon 1004 were placedin, a reactor fitted with a stirrer, thermometer, and heating mantle andheated with stirring to 350F. Then first the ER- b 0510 was added, andthen the Hycar CTBNX. The mixturewas heated again'to 350F. and held atthat temperature and stirred for 3 hours. During this reac- 'tion,theacid number of the mixture was reduced to zero, indicating completereaction between the elastomer and epoxy resin.'The mixture was thencooled to room temperature and dissolved into the methyl ethyl ketone.To the resulting solution was added a mixture that had been prepared bygrinding the dicyandiamide and reaction product of toluene 'diisocyanateand dimethylamine into the ERL-2774 on a three-roll paint mill.

Thissolut'ion was coated out in the manner described in Example 1 with ascrim cloth comprising a nylon tricot. The films were then tested by thestandard procedures described above with the following results:

Temperature of test 67F.

(mils) 4 9 The formulations of Examples 1 and 2 are preferred, butuseful bonding films may be provided with other materials and otherproportions". In general, most heatcurable epoxy resins having on theaverage more than one reactive 1,2 epoxy group' per molecule aresuitable for incorporation in the adhesive compositions of thisinvention, including aliphatic, aromatic and cycloaliphatic varieties.As the examples illustrate, several different epoxy resins can be usedin combination to provide a particular balance of properties. When theterm epoxy resin is used inthis specification it may meaneither oneepoxy resin or a combination of epoxy resins. Generally the epoxy resinor combination of epoxy resins is a room-temperature solid, althoughsome liquid varieties may be prereacted with elastomr orelastomer-precursor to form the needed highmolecular-weightflexibilizing polymer. To avoid brittleness in the cured bond throughexcessive crosslinking, the 'epoxy groups should be separated bynonreactive portions of the molecule. In general, the spacing will besufficient if the average epoxide-equivalent weight of the resin orresin blend is greater than about 200. Epoxy novolacs or other epoxyresins that have a high number of reactive groupsand provide. a-,-highcrosslink density in the cured bond may be .used in bonding films of theinvention and are especially useful to provide bonds of high performanceat elevated temperatures. y

The flexibilizing polymer in a bonding film of the invention maycomprise separate elastomer molecules, separate elastomer-precursormolecules, combination molecules that include epoxy-resin segments andelastomeric segments, or mixtures of such separate and combinationmolecules. The combination molecules may be prepared by reacting epoxyresin and material that provides elastomeric segments in the reactionproduct, the reaction leaving reactive functional groups, such asunreacted epoxy groups, on the reaction product. Whichever form theflexibilizing polymer takes, it must have a high averagemolecular-weight to obtain the desired level of strength properties.Usually the flexibilizing polymer is a room-temperature solid (though itmay within a few hours assume the shape of a container in which it isplaced). As will be subsequently shown, it has been found that theneeded molecular weight can generally be described by stating that asubstantial number of the molecules of flexibilizing polymer should havea molecular weight above about 8000; more specifically, theflexibilizing polymer should comprise sufficient molecules above about8000 in molecular weight to account for roughly at least about one-tenthof the weight of the whole composition.

While the reasons for better strengths are not fully understood, theneed for a high-molecular-weight flexibilizing polymer can be readilyshown. For example, 15 parts of a liquid low-molecular-weightcarboxylated acrylonitrile-butadiene elastomer-precursor simply mixedwith the rest of the ingredients in a bonding film will not producesatisfactory strength properties, while 15 parts of a solidhigh-molecular-weight carboxylated acrylonitrile-butadiene elastomerwill produce satisfactory strength properties. However, when a liquidlowmolecular-weight elastomer-precursor is properly prereacted with partof the epoxy resin in a composition to form a flexibilizing polymer ofhigh molecularweight, satisfactory strength properties are obtained.

The variation in properties obtained by varying the weight-percent offlexibilizing polymer that is above 8,000 in molecular weight is shownin Table 11. In the tests reported in the table, bonding films wereprepared using compositions in which the flexibilizing polymer variedfrom a liquid unreacted elastomer-precursor, through a series ofreaction products of different molecular weight prepared by prereactingelastomerprecursor with epoxy resin; in some cases the prereactionproduct was supplemented with solid elastomer. The elastomer-precursorand epoxy resin were prereacted in the general manner described inExample 2, but at different reaction temperatures and times of reaction.As can be seen from Table 11, higher temperatures and longer times ofprereaction lead to a larger weight-percent of flexibilizing polymerthat is above 8,000 in molecular weight.

The weight-percent of flexibilizing polymer above 8,000 molecular weightwas determined with a gel permeation chromatograph (GPC). The GPC usedwas a commercially available instrument manufactured by WatersAssociates (61 Fountain St., Framingham. Mass.). Briefly, thisinstrument separates polymeric species according to molecular size(which can be correlated with molecular weight) by passage of a dilutesolution (one-eighth to one-half percent) of the polymer of interestthrough a column packed with a uniformly crosslinked polystyrene gel.The abscissa on a typical chromatogram is usually calibrated in angstromunits, which refers to the average size of a species eluting at aparticular time. The ordinate is proportional to the weight of polymericspecies eluting at a particular time. By using standard calibrationtechniques, it was found that for the samples reported in Table 11, thespecies having a molecular weight of about 8,000 eluted at the 390Apoint on the abscissa of the chromatogram. The assignment of molecularweight values to a particular point on the abscissa scale is regarded asaccurate to within 1 25 percent.

The weight-percent of polymeric species above 8,000 in molecular weightin a sample tested was found by measuring the percentage of the area ofthe chromatogram curve above 390A. The total weight-percent of acomposition above 8,000 molecular weight was determined by assumingthat, except for the solid elastomer (Hycar 1072) added to some samplesin addition to the prereaction product, the other ingredients in thecomposition did not include material above 8,000 molecular weight.Ninety-five weight-percent of the solid elastomer was determined to beabove 8,000 molecular weight, and thus, for the compositions thatincluded solid elastomer, 95 percent of the amount of solid elastomer inthe composition was added to the weightpercent obtained from the GPCcurve.

TABLE 11 A* B C* D E F G H 1 .I K

Prereaction conditions Time (hrs) 1 1 1 0 l l 3 1 l 1 3 Temp (F.) 300300 300 300 300 350 300 300 350 350 Prereaction components Hycar 1300 X2 l5 Hycar CTBNX 1O 15 15 15 15 15 15 Epon 828 70 70 Epon 834 3O 30 3030 30 30 3O 30 Epon 1004* 40 40 4O 40 40 40 40 40 ERL-0510 15 15 15 1515 15 15 ERL-2774 15 15 Weight percent of prereaction product above I8000 molecular wt. 3.6 8.4 8.25 5.3 12.4 12.8 13.2 12.4 12.8 15.3 23.1Components added after prereaction Hycar 1072 5 5 5 5 ERL-2774 15 15 1515 15 15 15 ERL 0510 15 15 Dicyandiamide 3.3 4 3.3 4 4 4 4 4 4 4 4Reaction product of toluene diisocyanate and dimethylamine 6.6 8 6.6 8 88 8 8 8 8 3 Weight-percent of whole composition above about 8000 3.3 6.67.5 7.6 9.8 10.1 10.3 13.0 13.3 15.2 18.6

TABLE I1 Continued A* B C* D E F G H l .l K

Temperature of test Overlap shear (pounds/square inch) 67 F. 2207 47134237 2853 4430 4990 51 10 4223 4170 5316 6467 75 F. 4420 5286 2713 28864877 5313 5380 4110 4167 5340 5167 180 F. 2540 4412 2152 2520 3880 39864250 3330 3046 4060 3727 250 F. 602 1876 926 566 1370 1336 1550 1530 9901908 1520 T-peel (pounds/inch of width) 67 F 7 11.5 25 14**** l 18 2122.5 22 30 75 F. 7 21 7.5 26.5 25 25.5 28.5 27 27.5 27 27 180 P035513524 6 5 24.5 24.5 285 26.5 25 24.5 24 23 Honeycomb peel (inch-pound/inchof width) 67 F. 13.8 18.3 18.6 18 75 F. 9.5 21.6 16.2 19 180 F. 16.315.5 15.6 16

liquid composition applied to the surface of both adherends with a scrimbetween the adherends during curing a room temperature-liquidcarhoxylated acrylonitrile-butadiene elastomer having a numbenaveragemolecular weight of about 1000 and having about 0.08 acid equivalentsper 100 grams of material "a diglycidyl ether of bisphenol A having anepoxide-equivalent-weight of 875-1025 and a Durrans softening point of95-100C. Fcr reasons not fully understood. better results were obtainedfor this bonding film in the T-peel test when the surfaces bonded werenot primed; when the surfaces were primed, a value of pounds/inch widthwas obtained.

As shown in the table, the AD samples, in which the molecules offlexibilizing polymer above 8,000 molecular weight constitute about 7.6weight-percent or less of the whole composition, do not meet the minimumstrength tests established for high-performance sheetto-sheet andsheet-to-honeycomb members, while the samples F-K in which the amount ofhigh-molecularweight molecules of flexibilizing polymer is about 10.1weight-percent or more of the whole composition, do meet those tests.Sample E, in which the molecules above about 8,000 molecular-weightaccount for 9.8 weight-percent of the composition, is seen to be amarginal composition. However, satisfactory bonding films can be formedin which the number of molecules above about 8,000 molecular-weight issubstantially less than the number in sample E, for example, bymodifying the epoxy resin blend to use more flexible epoxy resins. Thus,the compositions that are satisfactory should not be defined byreference to a precise weight-percent point for the number of moleculesabove 8,000 molecular weight. But, in general, the needed molecularweight for the flexibilizing polymer will exist when roughly at leastone-tenth of the weight of the composition is accounted for themolecules of flexibilizing polymer that are above about 8,000 inmolecular weight.

The obtaining of the desired strength properties also depends on theamount of elastomeric segments in the bonding film. If the amount ofelastomeric segments in the composition falls below about 5 parts ofelastomeric segments per 100 parts of epoxy-resin segments, the curedbond is too brittle and peel strength properties are too low. On theother hand, if more than 40 resin segments are included in thecomposition, the strength properties of the cured bond also decreasebelow satisfactory levels. Preferred results are obtained when between10 and 35 parts of elastomeric segments are included for each parts ofepoxy-resin segments.

Table III illustrates the changes in strength properties that occur asthe amount of elastomeric segments in a bonding film approaches andexceeds 40 parts per 100 parts of epoxy-resin segments. This tablereports typical results of tests on eight different samples, AH,prepared using the same ingredients and proportions as used in Example1, except that the amount of elastomer (Hycar 1072) was varied fromsample to sample and, in addition, (a) the chrome oxide colorant wasomitted from all but the A sample, and (b) in samples Fl-l, a semi-solidnovolak epoxy resin having an epoxideequivalent weight of -182 (DEN-438)was used to increase the crosslink density of the cured product; thefinal blend of epoxy resin in samples F-l-l was 41.4 parts of Epon 834,9.6 parts of ERL2774, 36.7 parts of Epon 1004, and 12.2 parts of thenovolak epoxy. The tests reported in the table reveal that as the amountof elastomer increases: (1) overlap shear strengths at room-temperaturereach a peak at about 35 to 40 parts of elastomer added and thendecline; (2) overlap shear strengths at higher temperatures declinesteadily so that at 40 parts of elastomer the values are justacceptable; (3) room-temperature peel strengths steadily increase, buthigh-temperature peel strenghts steadily decline to reach marginalvalues at about 40 parts of elastomer added; and (4) beam-creep valuessteadily decline so that marginal values are obtained at parts ofelastomeric segments per 100 parts of epoxy- 55 40 parts of elastomeradded.

TABLE III A B C D E F G, H Temperaturc of test Hycar 1072 (parts byweight) 25 30 35 40 45 25 35 45 Film-weight for first two tests (poundsper square foot) 0.06 0.061 0.063 0.062 0.057 0.057 0.055 0.060 Overlapshear-strength (pounds/square inch) 67 F. 5652 6246 6680 6705 6317 59305597 6565 RT. 5690 5618 5486 5404 5130 5824 5780 5704 F. 3398 3134 28802702 2546 3408 2876 2486 250 F. 1266 1122 1082 1014 840 1194 908 716TABLE 111 -Continued A B C D E F G H Temperature of test T-pccl strength(pounds per inch width) -67 F. 25 24 30 33 26 23 26 RT. 37 40 42 46 4430 36 40 g 180 F; 21 18 18 16 16 26 19 17 Film-weight for last two tests(pounds/square foot) 0.090 0.092 0.095 0.093 0.086 0.086 0.083 0.090Honeycomb peel-strength (inch-pounds/inch width) -67 F. 31.6 35.0 41.031.7 25.0 24.2 35.0 R.T 38.2 41.0 42.4 44.2 36.6 32.5 31.6 38.2 180 F.47.5 33.2 29.5 23.8 19.5 37.6 21.6 20.6 Beam-creep (mils) 180 F. 25.0Not run 29.4 48.5 54.0 18.0 43 54.5 *170 hrs) (72 hrs.) (24 hrs.)

Flexibilizing polymers useful in the present invention may incorporate avariety of different kinds of reactive groups that permit theflexibilizing polymer to be linked to an epoxy resin and integrated intothe network of the cured bond. These reactive groups include carboxylgroups, mercapto groups, epoxy groups, and amine groups. Except perhapsfor epoxy groups, carboxyl groups are preferred reactive groups on aflexibilizing polymer because, except for the epoxy groups, they aremore latent in compositions of the invention.

Acrylonitrile-butadiene elastomeric segments are preferred incompositions of the invention because they are useful through a widerange of temperatures and have good chemical resistance. Another classof useful elastomeric segments include polyether diamines such aspoly(tetramethylene oxide) diamine described in U.S. Pat. No. 3,436,359.In general, the useful elastomer segments in compositions of theinvention are provided by chemically resistant, thermally stablematerials that may be introduced into the composition in a uniform,finely divided manner and maintain high strength properties and goodflexibility over the desired range of temperatures.

An additional example follows to illustrate a bonding film of theinvention that incorporates a poly(tetramethylene oxide) diamineelastomer-precursor.

EXAMPLE 3 The ingredients are as follows: Parts by Weight Epoxy resincomprising two diglycidyl ethers of bisphenol 100 A and the previouslydescribed ERL 0510 Durrans Epoxide Parts softening Equivalent by pointWeight Weight Epon 840 -68C. 330-380 33 Epon 1001 74C. 425-550 53.5 ERL0510 13.5

Parts by Weight 5 5 Poly(tetramethylene oxide) diamineelastomer-precursor 20 (A polyether having a number-average molecularweight of about 10,000 formed from tetrahydrofuran, withamine-termination on both ends of the polymer molecule; method forpreparation described in US. Pat. 3,436,359; this polyether may bevulcanized with epoxy resins to produce an elastomer) 0 Dicyandiamide2.6 Reaction product of toluene diisocyanate and dimethyl amine 8Solvents Toluene 66 Acetone 16 Methanol s 65 Tetrahydrofuran l 9 Methylethyl ketone 43 Dimethylforrnamide The poly(tetramethylene oxide)diamine and Epon 840 epoxy resin are prereacted by dissolving themtogether in the toluene, and then stirring the solution for 2 days atambient temperature. After the 2 days, the Epon 1001 and ERL 0510 epoxyresins and the methyl ethyl ketone are added to the solution and thecombined mixture is stirred to obtain a uniform solution. Thedicyandiamide and reaction product of toluene diisocyanate and dimethylamine are dissolved in the remaining solvents and added to the combinedsolution and the mixture stirred to obtain a uniform blend. Bondingfilms are then prepared by the method taught in Example 1 and the filmsare tested for their strength properties as follows:

The curing agent in a bonding film of the invention should be latent atroom temperature so as to permit storage of the bonding films forconvenient periods. (Bonding films of the invention are generally storedat 0F. to assure that there will be no prereaction.) Some preferredroom-temperature-latent curing agents are substituted urea curing agentsthat decompose and release an amine curing agent such as dimethyl aminewhen heated to an elevated temperature. The preferred ones decompose at250F.; these include 3-(p-chlorophenyl)-1,l-dimethyl urea and thereaction product of toluene diisocyanate and dimethyl amine, which wereused in Example 1-3, as well as 3-phenyl-l,1-dibutyl urea,3-phenyl-l-benzyl-l-methyl urea, and 3-phenyl- 1,1-pentamethylene urea.Other useful room-temperature-latent curing agents are dicyandiamide,the dihydrazides, and amine salts. The best strength properties inbonding films of the invention are obtained when dicyandiamide isincluded together with a substituted urea. Dicyandiamide gives goodstrength properties if it is the only curing agent, but requires curingof the bonding film at 350F.

Though curatives for a reactive elastomer such as carboxylatedacrylonitrile-butadiene elastomer are sometimes included to improve thefinal cure of adhesive compositions containing that elastomer forexample, see Martin et al., US. Pat. No. 2,951,004),room-temperature-active curatives such as iinc oxide are preferably notincluded in the composition of this invention. Such curing agentsprematurely cure the elastomer during storage, whereupon the bondingcomposition does not fillet well and strength properties of the curedproduct are reduced.

For use in bonding to honeycomb cores, the bonding film of the inventionshould have a thickness of at least about mils. Thinner films can beused in bonding planar-surfaced articles, or where less adhesive isrequired. On the other hand, the film generally need not have athickness greater than about 30 mils. A thin fabric such as a scrim istypically included in the film to improve the handleability of theproduct. lnert fillers may also be included in the film. The film isgenerally made available in a rather nontacky form and the film thenapplied with a tacking iron to a surface being bonded. However, the filmcan be supplied with substantial tackiness. The film is normallysupplied on a carrier sheet with a protective film over the surface ofthe film that is not against the carrier sheet.

What is claimed is:

1. A heat-curable self-supporting bonding film having a thicknessbetween about 5 and 30 mils and consisting essentially of a singlereactive layer that includes a heat-curable composition comprising (1)an epoxy resin which has on the average more than one reactive l,2 epoxygroup per molecule; (2) a high-molecularweight flexibilizing polymerthat (a) carries reactive functional groups for chemically linking theflexibilizing polymer to the epoxy resin, (b) provides 5 to 40 parts byweight of elastomeric segments in the composition for each 100 parts byweight of epoxy-resin segments, and (c) comprises sufficient moleculesthat have a molecular weight of at least 8,000 to account for at leastabout one-tenth of the weight of the composition; and (3) aroom-temperature-latent curing agent active at elevated temperature toinitiate reaction of the composition to an integrated cured state; saidbonding film being substantially free of room-temperature-active curingagents, and being further characterized in that upon heating to anelevated temperature it (a) becomes flowable so as to wet and form afillet on the edge portion of a honeycomb pressed against the film, and(b) subsequently cures to provide the highstrength bond necessary both(i) in a sheet-to-sheet structural member and (ii) in asheet-to-honeycomb structural member.

2. A bonding film of claim 1 in which the elastomeric segments providedby the flexibilizing polymer comprise acrylonitrile-butadieneelastomeric segments.

3. A bonding film of claim 1 in which the flexibilizing polymercomprises a prereaction product of an epoxy resin and a material thatprovides elastomeric segments in the prereaction product.

4. A bonding film of claim 1 in which the flexibilizing polymer providesbetween and 35 parts of elastomeric segments in the composition per 100parts of epoxy-resin segments.

5. A bonding film of claim 1 in which the curing agent comprises asubstituted urea that upon heating to about 250F. releases an amine thatcatalyzes crosslinking of epoxy resins.

6. A bonding film of claim 5 in which the curing agent further comprisesdicyandiamide.

7. A bonding film of claim 1 in which a scrim-type fabric is embedded inthe single reactive layer.

8. A heat-curable self-supporting bonding film having a thicknessbetween about 5 and 30 mils and consisting essentially of a singlereactive layer that includes a heat-curable composition comprising (1)an epoxy resin that has on the average more than one 1,2 epoxy group permolecule; (2) a flexibilizing polymer that (a) carries reactivefunctional groups for chemically linking the flexibilizing polymer tothe epoxy resin, (b) provides 5 to 40 parts by weight ofacrylonitrile-butadiene elastomeric segments in the composition for eachl00 parts by weight of epoxy-resin segments, and (0) comprisessufficient molecules that have a molecular weight of at least 8000 toaccount for at least about one-tenth of the weight of the composition;and (3) a room-temperature-latent curing agent active at elevatedtemperature to initiate reaction of the composition to an integratedcured state; said bonding film being substantially free ofroom-temperature-active curing agents for the flexibilizing polymer, andbeing further characterized in that upon heating to an elevatedtemperature it (a) becomes flowable so as to wet and form a fillet onthe edge portion of a honeycomb pressed against the film, and (b)subsequently cures to provide the high-strength bond necessary both (i)in a sheet-to-sheet laminated structural member and (ii) in asheet-to-honeycomb structural member.

9. A bonding film of claim 8 in which the flexibilizing polymer providesbetween l0 and 35 parts of elastomeric segments in the composition perparts of epoxy-resin segments.

10. A bonding film of claim 8 in which the curing agent comprises asubstituted urea that upon heating to about 250F. releases an amine thatcatalyzes curing of the composition.

11. A bonding film of claim 10 in which the curing agent furthercomprises dicyandiamide.

12. A bonding film of claim 8 in which the flexibilizing polymercomprises a room-temperature-solid reactive acrylonitrile-butadieneelastomer.

13. A bonding film of claim 8 in which the flexibilizing polymercomprises a prereaction product of epoxy resin and reactiveacrylonitrile-butadiene elastomerprecursor.

14. A bonding film of claim 8 in which the functional groups arecarboxyl, mercapto, epoxy, or amine groups.

15. A bonding film of claim 14 in which the room-temperature-latentcuring agent comprises a substituted urea that upon heating to about250F. releases an amine that catalyzes crosslinking of epoxy resins.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONPATENT NO. I 3 894 ,113

DATED I July 8, 1975 |NVENT0R(5) 1 Warren C. Pagel It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

In column 1, line 44, "deformat" should be --deformation-.

In Table II, the results in the T-Peel test for the A* sample should beas follows:

Signed and Scaled this [SEAL] Arrest:

RUTH C. MASON C. IAISIIALI. DANN Arresting Officer Commissioner 01km!and Trademarks thirtieth Day of December 1975

1. A HEAT-CURABLE SELF-SUPPORTING BONDING FILM HAVING A THICKNESSBETWEEN AND 5 AND 30 MILS AND CONSISTING ESSENTIALLY OF A SINGLEREACTIVE LAYER THAT INCLUDES A HEAT-CURABLE COMPOSITION COMPRISING (1)AN EPOXY RESIN WHICH HAS ON THE AVERAGE MORE THAN ONE REACTIVE 1,2 EPOXYGROUP PER MOLECULE, (2) A HIGH-MOLECULAR-WEIGHT FLEXIBILIZING POLYMERTHAT (A) CARRIER REACTIVE FUNCTIONAL GROUPS FOR CHEMICALLY LINKING THEFLEXIBILIZING POLYMER TO THE EPOXY RESIN, (B) PROVIDES 5 TO 40 PARTS BYWEIGHT OF ELASTOMERIC SEGMENTS IN THE COMPOSITION FOR EACH 100 PARTS BYWEIGHT OF EPOXY-RESIN SEGMENTS, AND (C) COMPRISES SUFFICIENT MOLECULESTHAT HAVE A MOLECULAR WEIGHT OF AT LEAST 8,000 TO ACCOUNT FOR A LEASTABOUT ONE-TENTH OF THE WEIGHT OF THE COMPOSITION, AND (3) AROOM-TEMPERATURE-LATENT CURING AGENT ACTIVE AT ELEVATED TEMPERATURE TOINITIATE REACTION OF THE COMPOSITION TO AN INTEGRATED CURED STATE, SAIDBONDING FILM BEING SUBSTANTIALLY FREE OF ROOM-TEMPERATURE-ACTIVE CURINGAGENTS, AND BEING FURTHER CHARACTERIZED IN THAT UPON HEATING TO ANELEVATED TEMPERATURE IT (A) BECOMES FLOWABLE SO AS TO WET AND FORM AFILLET ON THE EDGE PORTION OF A HONEYCOMB PRESSED AGAINST THE FILM, AND(B) SUBSEQUENTLY CURES TO PROVIDE THE HIGH-STRENGTH BOND NECESSARY BOTH(I) IN A SHEET-TO-SHEET STRUCTURAL MEMBER AND (II) IN ASHEET-TO-HONEYCOMB STRUCTURAL MEMBER.
 2. A bonding film of claim 1 inwhich the elastomeric segments provided by the flexibilizing polymercomprise acrylonitrile-butadiene elastomeric segments.
 3. A bonding filmof claim 1 in which the flexibilizing polymer comprises a prereactionproduct of an epoxy resin and a material that provides elastomericsegments in the prereaction product.
 4. A bonding film of claim 1 inwhich the flexibilizing polymer provides between 10 and 35 parts ofelastomeric segments in the composition per 100 parts of epoxy-resinsegments.
 5. A bonding film of claim 1 in which the curing agentcomprises a substituted urea that upon heating to about 250*F. releasesan amine that catalyzes crosslinking of epoxy resins.
 6. A bonding filmof claim 5 in which the curing agent further comprises dicyandiamide. 7.A bonding film of claim 1 in which a scrim-type fabric is embedded inthe single reactive layer.
 8. A heat-curable self-supporting bondingfilm having a thickness between about 5 and 30 mils and consistingessentially of a single reactive layer that includes a heat-curablecompositIon comprising (1) an epoxy resin that has on the average morethan one 1,2 epoxy group per molecule; (2) a flexibilizing polymer that(a) carries reactive functional groups for chemically linking theflexibilizing polymer to the epoxy resin, (b) provides 5 to 40 parts byweight of acrylonitrile-butadiene elastomeric segments in thecomposition for each 100 parts by weight of epoxy-resin segments, and(c) comprises sufficient molecules that have a molecular weight of atleast 8000 to account for at least about one-tenth of the weight of thecomposition; and (3) a room-temperature-latent curing agent active atelevated temperature to initiate reaction of the composition to anintegrated cured state; said bonding film being substantially free ofroom-temperature-active curing agents for the flexibilizing polymer, andbeing further characterized in that upon heating to an elevatedtemperature it (a) becomes flowable so as to wet and form a fillet onthe edge portion of a honeycomb pressed against the film, and (b)subsequently cures to provide the high-strength bond necessary both (i)in a sheet-to-sheet laminated structural member and (ii) in asheet-to-honeycomb structural member.
 9. A bonding film of claim 8 inwhich the flexibilizing polymer provides between 10 and 35 parts ofelastomeric segments in the composition per 100 parts of epoxy-resinsegments.
 10. A bonding film of claim 8 in which the curing agentcomprises a substituted urea that upon heating to about 250*F. releasesan amine that catalyzes curing of the composition.
 11. A bonding film ofclaim 10 in which the curing agent further comprises dicyandiamide. 12.A bonding film of claim 8 in which the flexibilizing polymer comprises aroom-temperature-solid reactive acrylonitrile-butadiene elastomer.
 13. Abonding film of claim 8 in which the flexibilizing polymer comprises aprereaction product of epoxy resin and reactive acrylonitrile-butadieneelastomer-precursor.
 14. A bonding film of claim 8 in which thefunctional groups are carboxyl, mercapto, epoxy, or amine groups.
 15. Abonding film of claim 14 in which the room-temperature-latent curingagent comprises a substituted urea that upon heating to about 250*F.releases an amine that catalyzes crosslinking of epoxy resins.